Ensure property-defined Vault token is used when auth is TOKEN

[johnycho]

Prior to this commit, SpringVaultTemplateBuilder ignored the configured static token if spring.cloud.config.server.vault.authentication was explicitly set to TOKEN. This caused the application to incorrectly fall back to default authentication mechanisms (e.g., filesystem token) even when a token was provided in the properties.

This change updates the isStaticToken predicate to return true when a token is present and the authentication method is either null (default) or explicitly set to TOKEN.

A unit test has been added to ensure the default ConfigTokenProvider is ignored in this scenario.

Fixes gh-3172

[osohyun0224]

@johnycho Wow! Respecting the explicitly configured token when authentication=TOKEN is set aligns much better with expected behavior, and the added unit test strengthens confidence in the change. I hope this gets merged :)

[ryanjbaxter]

Thank you. In the future please be sure to run the full build before submitting the PR to catch errors like this 11e1d45